The present invention relates to interferometric equipment for detecing a substance having a molecular structure such that it exhibits a periodic or quasiperiodic absorption spectrum, the equipment uses a source of radiation and a detector.
Copending application (Ser. No. 338,723, filed: 04/14/1989) corresponding to German application P38 12 334 describes interferometric equipment for detecting a substance with a periodic or quasiperiodic absorption spectrum using an electrically tunable interference filter. That filter is basically comprised of a plate made of an electrooptical material having its major flat sides provided with reflecting but semitransparent surface layers and boundaries. The thickness of the plate is selected to meet one of the following criteria.
The distance between the interference lines that are being produced by that particular filter equals a distance of the absorption lines within a periodic absorption spectrum of the substance to be detected; an alternative feature requires the thickness of the filter plate to be selected so that the bandwidth of an absorption line that can be separated is quite small as compared with the spacing between two absorption lines of that subject to be detected. A third alternative requires that a line as separated from the radiation will overlap an edge of the absorption band or of an absorption band of the substance to be detected.
The interferometric equipment of the kind described in the preceding paragarph can also be modified in that in lieu of a plate a cell which is filled with a double refracting liquid crystal filling is provided. Again however the windows are on the inside provided with a semitransparent reflective coating.
This particular equipment is highly useful and the principles employed have been deemed very successful, but there are certain substances where its employment seems less suitable. For example, if the presence or existence of carbon monoxide (CO) is selectively detected by using a wavelength of 4.6 micrometers, the following problems arise. For example, a lithium-niobate plate of appropriate thickness as far as the optical requirements are concerned can be selected, but electrical breakthrough may occur already at a controlled voltage as applied which is well below the voltage deemed optimum for purposes of the detection. In other words there is a discrepancy between the theoretical optimum sensitivity and a practical physical constraint for other reasons. On the other hand if a liquid crystal embodiment is used then the cell thicknesses that will be needed for obtaining resonance conditions are at values which are not adequately subject to tolerance control. Therefore these two kinds of equipment are not suitable for CO measurement. In fact similar difficulties are to be expected in all those cases in which the absorption lines have comparable spacing as compared with CO.